1. Field of the Invention
This invention relates to an angle measuring device, and in particular, to an angle measuring device for measuring the turning angle of a crankshaft in an internal combustion engine in order to control the ignition timing, etc. thereof.
2. Description of the Related Art
In order to control the ignition timing, etc. of an internal combustion engine, an angle measuring device is used which is adapted to emit a predetermined pulse in synchronization with rotations of the crankshaft of the engine. FIG. 1 shows an example of conventional devices of this type. The device shown comprises a shaft 1 which rotates in synchronization with the rotation of the crankshaft (not shown), a rotating disc 2 attached to the shaft 1, light-emitting elements 3 and 4, light-receiving elements 5 and 6, and an electronic circuit 7 including a semiconductor signal-processing circuit.
FIG. 2 is a plan view of the rotating disc 2 of FIG. 1. As shown in the drawing, a plurality of slits 8a and 8b are provided at predetermined intervals along the circumference of the rotating disc 2, respectively.
FIG. 3 is a circuit diagram schematically showing the signal processing section of the electronic circuit 7 of FIG. 1. The signal processing section shown includes a photodiode 9 one end of which is connected to a power-source line Vcc; this photodiode constitutes the light-receiving element 5 or 6 of FIG. 1. The reference numeral 10 indicates an equivalent capacitor between the anode and cathode of the photodiode 9; 11 indicates a resistor one end of which is connected to the anode of the photodiode 9 and the other end of which is grounded; and 12 indicates a signal processing circuit connected to a connection point A between the photodiode 9 and the resistor 11.
FIG. 4 is a chart showing the quantity of light and the voltage waveform at the connection point A when the photodiode 9 of FIG. 3 receives light emitted from the light-emitting elements 3 and 4.
In this conventional angle measuring device, which has the above-described construction, the rotating disc 2 shown in FIG. 2 rotates by being driven by the shaft 1, in synchronization with the rotation of the associated crankshaft. Light is input to the light-receiving elements 5, 6 and causes them to emit signals only when slits 8a and 8b are positioned between them and the light-emitting elements 3, 4. These signals are processed in the signal processing circuit shown in FIG. 3. Referring to FIG. 3, an output current in accordance with the incident quantity of light flows through the photodiode 9. When the incident quantity of light increases, this output current is augmented, so that the photodiode 9 is forward-biased by the voltage generated in the resistor 11. As a result of this forward-biasing, the capacitor 10 is charged with the time constant determined by the resistor 11 and this equivalent capacitor 10. When the incident quantity of light decreases, the capacitor 10 discharges with the above-mentioned time constant, so that the voltage at the connection point A becomes OFF at an instant t2 which is delayed by D with respect to the instant t1 at which the incident light becomes OFF, as shown by the pulse waveform indicated by the dashed line of FIG. 4.
As a result of this delay D, the accuracy of the signals processed in the signal processing circuit 12 deteriorates, which causes the accuracy of the turning angle of the crankshaft measured by the signal processing circuit 12 to deteriorate.